Various types of liquid coating compositions have been applied to substrates such as metallic substrates and baked thereon in order to protect substrates against degradation and corrosion. The coatings deposited on the substrates must generally be sufficiently adherent and flexible to resist cracking, chipping and peeling. The coating also must be resistant to staining and soiling, and it is also desirable that the coated substrate be recoatable with one or more additional coatings. The ability of the initial coating to provide a good adhesive bond with the second and subsequent coatings is an important consideration when evaluating coating compositions.
A variety of resin materials have been utilized as coating compositions on various substrates, and the coating resins include polyester resins, acrylic resins, alkyd resins, vinyl resins, etc. The present invention relates particularly to coating compositions utilizing polyester or alkyd resins. Polyester and alkyd resin coating formulations will contain in addition to the resin, one or more pigments, pigment extenders, suspending agents, binders, thinners, dryers, vehicles, flow aids, etc., to improve application properties as well as to improve the appearance of the coating applied to various substrates. Many materials have been suggested in the prior art for use in polyester and alkyd resin coating compositions to improve the application properties and film appearance, and many of these can be utilized in the coating compositions of the present invention.
Flow aids are incorporated into coating compositions such as paint formulations to reduce surface defects in paint films and to contribute to a smooth, level, even, and uniform film. Film defects can be caused by air bubbles in the applied paint film, by dust, dirt, oil, or other contaminants in the film or on the substrate which, as a result of differences in surface tensions, cause the paint to pull away from the contaminant resulting in film defects known as "craters", or "fish-eyes". Materials useful as flow aids function by migrating to the surface of a paint film after the film has been applied to a substrate where the flow aids lower surface tension so that air bubbles are broken and/or are capable of leaving the paint film, and when the surface tension of the paint film has been lowered by the flow aid, the paint does not pull away from contaminants thereby resulting in a smooth, level and generally defect-free coating.
Flow aids based on acrylic polymers have been utilized in the prior art. For example, Modaflow is a commercial acrylic flow aid available from Monsanto. This and other commercially available acrylic flow aids are effective in improving the surface characteristics of coatings such as paints, but in some instances, coatings and paints utilizing such acrylic flow aids have a tendency to attract and collect dirt thereby resulting in stains and generally unsatisfactory appearance. Moreover, coatings and paints containing some presently available acrylic flow aids are often characterized by unsatisfactory recoatability. Silicon-type flow aids, for example, are known for their poor recoatability.
Prior art acrylic polymer compositions which have been suggested as being useful as flow aids in various coating and paint formulations generally are soft, low Tg (glass transition) polymers based on long chain acrylate or methacrylate esters such as butyl acrylate, lauryl methacrylate, stearyl methacrylate, 2-ethylhexyl acrylate, etc.